This invention relates to coaxial electric cables, and more particularly to flexible coaxial cables suitable for carrying microwave signals with extremely low attenuation and extremely low radiation loss.
In a conventional coaxial cable, a center conductor is surrounded by a dielectric which is in turn surrounded by an outer conductive shield serving as the outer conductor generally coaxial with the center conductor. In conventional flexible coaxial cables, this outer shield is formed by a braid of electrical wires. In some flexible cables a second braided shield surrounds the first and the composite is called a double shield braid.
Coaxial cables with such braided shields are suitable for lower frequency applications. However, at higher frequencies, i.e. above approximately 10 gigaHertz (GHz) the attenuation per hundred feet of length of such conventional flexible coaxial cables becomes unacceptable for many communicative applications. For example, in a standard Mil. Spec. RG142-Type cable with a double shield braid having a 50 ohm impedance the loss is approximately 62 decibels (dB) per 100 feet at 10 gigaHertz, and if the frequency were to be increased further up to 12.4 GHz, the attenuation for such conventional double braided coaxial cable might rise to as high as approximately 80 dB per 100 feet. This unduly high attenuation makes conventional braided coaxial cables, even those with double shield braid, unsuitable for high frequency applications much above 10 GHz. In fact, this effective frequency limitation for such conventional braided coaxial cable is so well know and accepted in practice that catalogue information thereon does not go above approximately 12.4 GHz.
Another problem with conventional braided shield coaxial cable is that the cross-over regions where the strands of the braid weave over-and-under each other create "windows" or openings in the braid through which electrical energy "leaks" or radiates away from the cable. In other words, because of such windows or openings, the braided shield covers significantly less than 100% of the area of the outside surface of the dielectric medium. Even when multiple braid layers are employed, the signal can leak out through the windows in the inner braid layer, travel along between the braid layers and then leak out through the windows in the outer braid layer. Also, such windows allow "crosstalk" to occur between neighboring cables carrying different signals; that is, some of the signal energy leaks or radiates out of a first cable and into a second cable, thereby mixing with the signal being carried by the second cable. This crosstalk problem can be serious if the energy level of the signal in the first cable is much higher than that of the signal in the second cable.
A further problem with conventional braided shield coaxial cable is that during flexing the individual strands in the cable move or shift in position with respect to their neighbors, thereby creating rubbing contact which generates electrical noise including high frequency components of noise which undesirably mix with the signal being carried by the cable.
Attempts have been made to defeat leakage and radiation losses and crosstalk by including one or more layers of conductive foil or conductive coated Mylar associated with the layers of the multiple shield braids comprising the outer conductor. The inclusion of such foil does reduce the effects of leakage, radiation loss and crosstalk, but the resultant cable is increased in diameter and is relatively stiff and fragile, being subject to failure by rupture tearing or creasing of the foil. Moreover, the resultant increase in diameter causes the use of more silver-plated copper wire to form the braid layers.
There is a semi-rigid type of coaxial cable which is used in transmitting high frequency signals. Such semi-rigid type cables generally include a cylindrical copper tube as the outer conductor. Such cables lack the flexibility of the braided coaxial cables, but they do provide a lesser attenuation, for example, about 60 db at 18 GHz for a 50 ohm, Mil. Spec. 141-Type semi-rigid coaxial cable with a tubular copper shield.